Just following on from last post.

I went into panic mode when I read the Opening Post and thought likewise - might as well give up selling now then as I also use 99.9% large letter, so I rang my local PO and was told, definitely not - there is a special Express envelope that can only be used for documents, but other than that, so long as the padded envelopes fit through the 20mm slot there is no problem - phew, huge sigh of relief from me as I trust my local PO 110%.

Can only suggest as someone else has already, try a different PO if that is possible.